Anode catalysts used for polymer electrolyte fuel cells are mainly platinum and platinum-alloy-based catalysts. Specifically, catalysts in which a platinum-containing noble metal is supported by carbon black have been used. In terms of practical applications of polymer electrolyte fuel cells, one problem relates to the cost of materials. A means to solve such problem involves reduction of the platinum content.
Meanwhile, it has been known that when oxygen (O2) is electrolytically reduced, superoxide is generated as a result of one-electron reduction, hydrogen peroxide is generated as a result of two-electron reduction, or water is generated as a result of four-electron reduction. When voltage drop occurs for some reason in a fuel cell stack using, as an electrode, a platinum or platinum-based catalyst, four-electron reduction performance deteriorates, resulting in two-electron reduction. Accordingly, hydrogen peroxide is generated, causing MEA deterioration.
Recently, low-cost fuel cell catalysts for a reaction that produces water as a result of four-electron reduction of oxygen have been developed, which will result in elimination of the need for expensive platinum catalysts. Electrochimica Acta, vol. 39, No. 11/12, pp. 1647-1653, 199 discloses that a catalyst comprising a chalcogen element is excellent in terms of four-electron reduction performance and suggests that such catalyst be applied to fuel cells.
Likewise, JP Patent Publication (kohyo) No. 2001-502467 A discloses, as a platinum (Pt) catalyst substitute, an electrode catalyst composed of at least one transition metal and a chalcogen in which an example of a transition metal is Ru and an example of a chalcogen is S or Se. It is also disclosed that, in such case, the Ru:Se molar ratio is from 0.5 to 2 and the stoichiometric number “n” of (Ru)nSe is 1.5 to 2.
Further, JP Patent Publication (kohyo) No. 2004-532734 A discloses, as a Pt catalyst substitute, a fuel cell catalyst material comprising a transition metal that is either Fe or Ru, an organic transition metal complex containing nitrogen, and a chalcogen component such as S.
Further, J. Chem. Soc., Faraday Trans., 1996, 92 (21), 4311-4319 discloses Ru—S, Mo—S, and Mo—Ru—S binary and ternary electrode catalysts and methods for synthesizing the same.
Further, Electrochimica Acta, vol. 45, pp. 4237-4250 2000 discloses Ru—Mo—S and Ru—Mo—Se ternary chalcogenide electrode catalysts.